README: Add details on example usage and recommended configuration de…

…faults.

README.md

@@ -2,6 +2,7 @@
 
 A data compression/decompression library for embedded/real-time systems.
 
+
 ## Key Features:
 
 - **Low memory usage (as low as 50 bytes)**      
@@ -15,18 +16,105 @@ A data compression/decompression library for embedded/real-time systems.
 - **ISC license**      
     You can use it freely, even for commercial purposes.
 
+
 ## Getting Started:
 
 There is a standalone command-line program, `heatshrink`, but the
 encoder and decoder can also be used as libraries, independent of each
 other. To do so, copy `heatshrink_common.h`, `heatshrink_config.h`, and
 either `heatshrink_encoder.c` or `heatshrink_decoder.c` (and their
-respective header) into your project.
+respective header) into your project. For projects that use both,
+static libraries are built that use static and dynamic allocation.
 
 Dynamic allocation is used by default, but in an embedded context, you
 probably want to statically allocate the encoder/decoder. Set
 `HEATSHRINK_DYNAMIC_ALLOC` to 0 in `heatshrink_config.h`.
 
+
+### Basic Usage
+
+1. Allocate a `heatshrink_encoder` or `heatshrink_decoder` state machine
+using their `alloc` function, or statically allocate one and call their
+`_reset` function to initialize them. (See below for configuration
+options.)
+
+2. Use `sink` to sink an input buffer into the state machine. The
+`input_size` pointer argument will be set to indicate how many bytes of
+the input buffer were actually consumed. (If 0 bytes were conusmed, the
+buffer is full.)
+
+3. Use `poll` to move output from the state machine into an output
+buffer. The `output_size` pointer argument will be set to indicate how
+many bytes were output, and the function return value will indicate
+whether further output is available. (The state machine may not output
+any data until it has received enough input.)
+
+Repeat steps 2 and 3 to stream data through the state machine. Since
+it's doing data compression, the input and output sizes can vary
+significantly. Looping will be necessary to buffer the input and output
+as the data is processed.
+
+4. When the end of the input stream is reached, call `finish` to notify
+the state machine that no more input is available. The return value from
+`finish` will indicate whether any output remains. if so, call `poll` to
+get more.
+
+Continue calling `finish` and `poll`ing to flush remaining output until
+`finish` indicates that the output has been exhausted.
+
+Sinking more data after `finish` has been called will not work without
+calling `reset` on the state machine.
+
+
+## Configuration
+
+heatshrink has a couple configuration options, which impact its resource
+usage and how effectively it can compress data. These are set when
+dynamically allocating an encoder or decoder, or in `heatshrink_config.h`
+if they are statically allocated.
+
+- `window_sz2`, `-w` in the CLI: Set the window size to 2^W bytes.
+
+The window size determines how far back in the input can be searched for
+repeated patterns. A `window_sz2` of 8 will only use 256 bytes (2^8),
+while a `window_sz2` of 10 will use 1024 bytes (2^10). The latter uses
+more memory, but may also compress more effectively by detecting more
+repetition.
+
+The `window_sz2` setting currently must be between 4 and 15.
+
+- `lookahead_sz2`, `-l` in the CLI: Set the lookahead size to 2^L bytes.
+
+The lookahead size determines the max length for repeated patterns that
+are found. If the `lookahead_sz2` is 4, a 50-byte run of 'a' characters
+will be represented as several repeated 16-byte patterns (2^4 is 16),
+whereas a larger `lookahead_sz2` may be able to represent it all at
+once. The number of bits used for the lookahead size is fixed, so an
+overly large lookahead size can reduce compression by adding unused
+size bits to small patterns.
+
+The `lookahead_sz2` setting currently must be between 3 and the
+`window_sz2` - 1.
+
+- `input_buffer_size` - How large an input buffer to use for the
+decoder. This impacts how much work the decoder can do in a single
+step, and a larger buffer will use more memory. An extremely small
+buffer (say, 1 byte) will add overhead due to lots of suspend/resume
+function calls, but should not change how well data compresses.
+
+
+### Recommended Defaults
+
+For embedded/low memory contexts, a `window_sz2` in the 8 to 10 range is
+probably a good default, depending on how tight memory is. Smaller or
+larger window sizes may make better trade-offs in specific
+circumstances, but should be checked with representative data.
+
+The `lookahead_sz2` should probably start near the `window_sz2`/2, e.g.
+-w 8 -l 4 or -w 10 -l 5. The command-line program can be used to measure
+how well test data works with different settings.
+
+
 ## More Information and Benchmarks:
 
 heatshrink is based on [LZSS], since it's particularly suitable for
@@ -44,6 +132,7 @@ documentation.
 [index]: http://spin.atomicobject.com/2014/01/13/lightweight-indexing-for-embedded-systems/
 [LZSS]: http://en.wikipedia.org/wiki/Lempel-Ziv-Storer-Szymanski
 
+
 ## Build Status
 
   [![Build Status](https://travis-ci.org/atomicobject/heatshrink.png)](http://travis-ci.org/atomicobject/heatshrink)